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United States Patent |
6,215,387
|
Person
,   et al.
|
April 10, 2001
|
Thick film low value high frequency inductor
Abstract
A thick film low value high frequency inductor made by the process of
subjecting a conductor layer to a plurality of linear cuts by a pulsing
laser cutter imposed simultaneously on the entire length of the linear cut
being made to create a cross sectional cut of substantial rectangular
configuration. The conductor body is a layer of dried silver thick film
ink. The method of making a thick film low value high frequency inductor
involves the steps of taking a conductor layer comprised of a dried layer
of photo sensitive silver ink, masking the ink with the negative of the
desired configuration of the ink, exposing the ink to UV radiation,
developing the ink, and firing the layer to adhere the silver to the
layer.
Inventors:
|
Person; Herman R. (Columbus, NE);
Veik; Thomas L. (Columbus, NE);
Adelman; Jeffrey T. (Columbus, NE)
|
Assignee:
|
Vishay Dale Electronics, Inc. (Columbus, OH)
|
Appl. No.:
|
080494 |
Filed:
|
May 18, 1998 |
Current U.S. Class: |
336/200; 336/233 |
Intern'l Class: |
H01F 005/00 |
Field of Search: |
219/121.69,121.73,121.85
29/602.1
427/556
336/200,223
257/531
|
References Cited
U.S. Patent Documents
3874075 | Apr., 1975 | Lohse | 29/602.
|
4016519 | Apr., 1977 | Haas.
| |
4494100 | Jan., 1985 | Stengel et al. | 336/200.
|
4497020 | Jan., 1985 | Gilligan | 219/121.
|
4626816 | Dec., 1986 | Blumkin et al.
| |
4730095 | Mar., 1988 | Richter-Jorgensen | 219/121.
|
4905358 | Mar., 1990 | Einbinder | 219/121.
|
4970780 | Nov., 1990 | Suda et al.
| |
5047296 | Sep., 1991 | Miltenberger et al. | 428/694.
|
5071509 | Dec., 1991 | Kano et al. | 216/18.
|
5091286 | Feb., 1992 | Person.
| |
5504986 | Apr., 1996 | Casey et al. | 219/121.
|
5639391 | Jun., 1997 | Person | 219/121.
|
5647966 | Jul., 1997 | Uriu et al. | 205/78.
|
5809634 | Sep., 1998 | Inaba | 29/603.
|
Foreign Patent Documents |
2263582 | Jul., 1993 | GB.
| |
63-73606 | Apr., 1988 | JP.
| |
1-21993 | Jan., 1989 | JP.
| |
4-73917 | Mar., 1992 | JP.
| |
Primary Examiner: Heinrich; Samuel M.
Attorney, Agent or Firm: Zarley, McKee, Thomte, Voorhees & Sease
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional of applicant's application Ser. No.
08/936,193 filed Sep. 17, 1997, now U.S. Pat. No. 5,922,514.
Claims
What is claimed is:
1. A thick film inductor made by the process of subjecting a conductor
layer to a plurality of linear cuts by a pulsing laser cutter imposed
simultaneously on the entire length of the linear cut being made to create
a cross sectional cut of substantially rectangular configuration, wherein
the laser cutter converts the conductor layer into a conductor coil, the
conductor coil having lines with widths of less than about 2 mils, and the
lines having a thickness of at least about 15 microns.
2. The device of claim 1 wherein said conductor layer is composed of a
layer of dry silver thick film ink.
3. The thick film inductor of claim 1 wherein the distance between adjacent
conductor lines is less than about 2 mils.
4. The thick film inductor of claim 1 wherein the process further comprises
screen printing thick film conductor material onto a substrate to form the
conductor layer.
5. A thick film inductor, comprising:
a ceramic substrate;
a conductor coil, which has a cross section, positioned on the ceramic
substrate, the conductor coil comprised of lines whose widths are less
than about 2 mils, the lines having a thickness of at least about 15
microns, the cross section of the conductor coil having a substantially
rectangular configuration.
6. The thick film inductor of claim 5, wherein the ceramic substrate is
alumina.
7. The thick film inductor of claim 5 further comprising:
a dielectric base coat between the ceramic substrate and the conductor
coil, the dielectric base coat having a dielectric constant of no greater
than about 5.
8. The thick film inductor of claim 5 wherein the distance between adjacent
conductor lines is less than about 2 mils.
9. The thick film inductor of claim 5 wherein the thickness of the
conductor lines is between about 15 and 25 microns.
Description
FIELD OF THE INVENTION
This invention relates to the design and construction of low value, high
frequency inductors. These parts are particularly suited for the
communications industry. The current trend is toward continued
miniaturization and increased frequency.
BACKGROUND OF THE INVENTION
Typical prior art inductors use one of two manufacturing processes. 1) Thin
Film or 2) Individually spiraled copper plated ceramic cores.
The Thin Film process requires large capital expenditures. Additionally,
the time to build up conductor thicknesses sufficient to meet Q
requirements is long, keeping manufacturing costs high.
The second process, spiraled parts, has very high manufacturing costs due
mainly to the necessity to handle parts individually. Individual part
handling increases manufacturing time and costs dramatically.
Q is a measure of quality in inductors. It is the ratio of inductive
reactance, to the sum of all resistive losses. Inductive reactance is
desirable. Resistive losses, one of which is skin effect, are undesirable.
Skin effect is the tendency for alternating current to flow near the
surface of conductors in lieu of flowing in a manner as to utilize the
entire cross sectional area of conductors. This phenomenon causes the
resistance of the conductor to increase, thus reducing Q. When the
conductor cross section includes sharpened areas at each side, skin effect
is even worse. Existing equipment and methods diminish the Q value because
the cross sectional cut of conductor layers in fabrication create cross
sectional cuts of non-rectangular configuration.
A high frequency chip inductor on ceramic is currently made by
screenprinting thickfilm conductor ink forming fine lines on a ceramic
substrate. An alternative method is to expose photoimageable conductor ink
to UV radiation through a mask, and then develop the pattern via wet
chemistry processing. However, these two technologies limit the number of
turns in the spiral because they are limited to making line widths no
finer than about 4 mils.
It is therefore a principal object of this invention to provide a thick
film low value high frequency inductor which is low in resistive losses
including skin effect.
A further object of this invention is to provide a thick film low value
high frequency inductor, and a method of manufacturing the same which does
not have sharpened areas at its side edges whereupon skin effect is
reduced.
A still further object of the invention is to use a pulsing laser cutting
technique wherein the entire cut is performed simultaneously which
shortens the cutting time.
A still further alternative object of this invention is to provide a method
of making an inductor using a photosensitive silver ladened printing ink
exposed to ultra violet light in combination with a suitable negative
image mask to create the desired image.
A still further object of this invention is to provide a chip inductor
comprised of a multi-turn spiral coil on the upper surface of a
nonmagnetic dielectric substrate.
A still further object of this invention is a chip inductor comprised of a
spiral coil which is formed by ablating a spiral coil pattern into a
conductive layer on a substrate by the use of a laser beam.
A still further object of this invention is a chip inductor coil formed by
ablating a spiral coil pattern into a conductive layer on a substrate by
the use of an excimer laser operating in the ultraviolet region of the
electromagnetic spectrum.
A still further object of this invention is to provide for a substrate base
layer that has a low dielectric constant of around 4, which is
considerably lower than alumina, which is around 9.
A still further object of this invention provides for a substrate made of
alumina and has a layer of a low dielectric constant dielectric covering
the upper surface of said alumina substrate and underlays said conductive
layer.
A still further object of this invention provides for an excimer laser that
can ablate the conductive layer to provide the shape of a spiral
conductive coil without cutting into the low dielectric constant
dielectric layer on the upper side of the alumina substrate or into the
alumina substrate itself.
A still further object of this invention provides for a second spiral coil
to be positioned above the first spiral coil on the substrate with a layer
of low K dielectric between the two coils to keep said coils spaced apart
but with a via hole positioned to connect one end of each coil to provide
for two coils in series.
These and other objectives will be apparent to those skilled in the art.
SUMMARY OF THE INVENTION
A laser pulsing technique of the type disclosed in U.S. Pat. No. 5,091,286
is used to laser cut the coil image in a conductive coil. The low pulsing
technique of the '286 patent is incorporated herein by reference. This
process yields conductor cross sections that are more rectangular than in
conventional thick film screen printing.
This invention involves a new and improved method for manufacturing a high
frequency chip inductor on ceramic.
This new method allows achieving higher inductance values by making line
widths much finer than the 2 mils allowed by the prior art. Finer lines
permitting more loops than allowed by the prior art.
This new method consists of using the ultraviolet output of an excimer
laser to cut a planar spiral coil pattern in conductor material that is in
the form of a flat layer on a substrate. However, rather than making
linear cuts with a fine focused, scanned beam (as would be done in a
typical industrial metal cutting operation), a beam with a large cross
section, approximately 4 cm by 3 cm, is projected through a metal
"stencil" mask that contains the coil pattern. This transforms the beam's
uniformly intense cross section into a cross section containing the coil
pattern. The transformed beam is then optically focused onto a blank metal
target into which the coil pattern is burned or permanently transfixed by
the process of ablation. The UV excimer laser has a pulsed output so that
one or more pulses will be required to produce a clean and clear coil
pattern in the conductor material. The number of pulses required will
increase with the thickness of the conductor. Suitable conductor targets
on ceramic can be made by thickfilm printing, thinfilm deposition, or by
bonding metal foil to the ceramic surface.
An alternate form of the invention creates the rectangular cross sectional
cuts of conductor layers by a special film screening process which
involves the steps of taking a ceramic sheet of high (very hard) alumina
which is pre-scribed so that it can be broken into small inductors. An
organic layer of silver ladened ink is printed thereon, and then dried. A
photo mask that is a negative of the desired image is placed over, and in
contact with, the dried photoimagable ink. This configuration is next
exposed to UV radiation, polymerizing the ink not masked by the photo
mask. Next the photo mask is removed and the "ink" is then chemically
developed, the nonpolymerized material is washed away and the desired
silver image remains. The device is then fired, the non-developed material
is washed away and the silver image remains. Appropriate crossovers
between images can thereafter be printed. This process produces a
conductor layer with rectangular cross sections and which provides
conductors with line, widths and spacings smaller than can be made with
conventional thick film screen printing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a plan view of chip inductor excluding dielectric overcoat;
FIG. 2A is an end termination print on back side of substrate;
FIG. 2B is a dielectric base coat on top side of substrate;
FIG. 2C is a coil print;
FIG. 2D is a crossover dielectric;
FIG. 2E is a crossover conductor;
FIG. 2F is a dielectric overcoat;
FIG. 3A is an end view of strips of chip inductors (edge view of individual
component) immediately after breaking into strips;
FIG. 3B is an end view of strips of chip inductors after applying edge
termination;
FIG. 4 is a plan view of a strip of chip inductors ready for edge
termination;
FIG. 5A is an alternate crossover dielectric with via hole;
FIG. 5B is an alternate crossover conductor;
FIG. 5C is a coil and alternate crossover conductor showing relative
positions;
FIG. 6A is a cross section of a conventional prior art screen printed
conductor; and
FIG. 6B is a cross section of silver conductor made according to this
invention as taken on line 6B--6B of FIG. 5B.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Before describing the process of manufacturing the inductors, it should be
understood that hundreds or even thousands of individual parts are formed
on each substrate. Each substrate will hold a rectangular array of chip
inductors. The substrate material is alumina, and has been scored on its
top surface by its manufacturer at the boundaries of the individual
inductors that will be printed thereon. The scoring is an aid in breaking
the substrate into individual inductors when all screen printing steps
have been completed. Scoring of the bottom side of the substrate is
optional.
Each pattern is thick film screen printed with the appropriate inks. Each
individual print includes the required alignment marks for proper print to
substrate, and print to print alignment.
The first print, shown in FIG. 2A, is the silver bottom side termination.
It is printed on the bottom, non-scored side of the substrate. Bottom side
scoring is optional.
The second print, shown in FIG. 2B, is a dielectric base having a low
dielectric constant. One very important quality of low value, high
frequency inductors is high Self Resonant Frequency (SRF). To achieve
this, the inductor must be manufactured using materials with the lowest
dielectric constant possible. The dielectric constant of the alumina
substrate is excessively high, around 9. Covering the alumina substrate
with a dielectric base coat with a lower dielectric constant, around 4,
helps to keep the Self Resonant Frequency of the inductor as high as
possible.
FIG. 2C is the silver inductor coil resulting from the process of this
invention. After the completion of the coil in FIG. 2C, a dielectric
material is screen printed over the coil to provide for a crossover
dielectric layer shown in FIG. 2D. Its function is to prevent electrical
shorting between the inductor coil and the silver crossover printed
thereon.
FIG. 2E illustrates the silver crossover print. The silver coil of FIG. 2C
has a first end 12 connected to a first termination 10. It is the function
of the silver crossover to electrically connect the second end of the
silver coil 13 to the second termination 11.
The next print, FIG. 2F, is the dielectric overcoat. This material, like
all the other dielectric materials, must have a dielectric constant as low
as possible, preferably less than 4-5. The dielectric overcoat protects
the inductor from environmental factors.
At this point, all the physical features for a single spiral coil chip
inductor have been described. There are two different technologies (to be
described later in this application) that can be used to fabricate the
single spiral inductor. One technology uses a photoimageable thickfilm
conductor composition, such as a modification of DuPont's FODEL.RTM.
system. The FODEL.RTM. ink utilizing silver and palladium has been
modified herein to use only silver. The other, alternate technology
requires the use of an excimer laser, which operates in the near
ultraviolet, to ablate layers of conventional thickfilm conductor
materials. Such conventional conductor materials are available from
vendors such as Heraeus-Cermalloy, DuPont, Electroscience Labs, Thick Film
Systems, Inc., and so forth.
A second viable form of this invention consists of having two spiral coils,
one on top of the other, rather than just a single spiral coil. Having two
spiral coils instead of one provides a significant increase in the
inductance. This alternate design with two coils necessitates some minor
changes in the patterns of FIG. 2. Such alternate patterns are now herein
described.
FIG. 5A shows a crossover dielectric 14, providing more area coverage, that
would be used instead of the crossover dielectric 5 of FIG. 2D. FIG. 5B
illustrates the second coil 15, that would be used instead of the
crossover conductor 6 of FIG. 2E. The electrical connection between end 13
of coil 4 (FIG. 2C) and end 17 of coil 15 (FIG. 5B) is accomplished
through via hole 16 (FIG. 5A).
FIG. 5C shows the second spiral coil 15 over coil 4.
Note that the alternate crossover dielectric 14 is not shown because it
would hide coil 4. Note in FIG. 5C as well as FIG. 1, the conductor
overlap is kept to a minimum, in order to minimize the conductor to
conductor capacitance and, hence, maximize the self resonant frequency.
After this second coil is formed, then a top dielectric coating is applied
in the same way as would be done for the single spiral coil inductor.
After all the desired patterns are formed and the top coatings applied,
then the next step is to break the substrates into strips as shown in FIG.
4. This prepares the chip inductors for edge termination. FIG. 3A is an
end view of a strip of chip inductors. Note that the alumina substrate 1
is exposed between bottom terminations 2 and end terminations 10, 11, at
the edge of the strips. Next, a very thin layer of metal 8 is sputtered
onto the strip edges as shown in FIG. 3B to prepare the edge for plating.
Following this, a solderable termination 9 is plated on the edges of the
strips. The next step is to break the strips into individual chip
inductors. The breaks will of course occur along the scribes in the
alumina substrate.
PRIOR ART
The prior art simply uses standard and, commonly known, thickfilm screen
printing techniques to print spiral coils, crossover dielectrics,
crossover conductors, and top or overcoat dielectric materials. There are
several major disadvantages to the prior art in that: 1) conductor line
widths and spaces between said lines cannot be printed reliably and
repeatedly small than about 0.006". Also, the conductor profile or
cross-section do not have sharp, square edges at the substrate interface,
but will show a feathering or a sloping of the edge, as shown in FIG. 6A.
This deviation from a vertical edge surface is believed to contribute
substantially to the AC or high frequency resistance that will adversely
affect the quality, or Q, of the chip inductor, as shown in FIG. 6B. With
a more vertical edge and square sides, the electrical properties of the
chip inductor are greatly improved and enhanced. The two, alternate
methods to be described herein provide the capability to produce spiral
coils whose edges are sharp and square, and sides which are vertical that
minimize undesirable AC effects.
PHOTOIMAGABLE PROCESS
The first process uses a photoimagable, photosensitive conductive thick
film composition. Ink material for the process is a modification, as
explained above, of the DuPont Company FODEL.RTM. ink, and hereafter will
be referred to as the photoimagable process. The photoimagable process
generally consists of printing a blank or uniform layer of the
photoimagable silver ladened conductive ink onto the substrate while under
a safe light. After drying, a photonegative, usually made of mylar and
with the coil pattern, is positioned in intimate contact onto the
conductor layer and carefully aligned with the aid of fudicials or
alignment marks by a mask aligner. The alignment is very important because
each coil must be precisely located. The photoimagable conductor material
is then exposed to an intense flux of ultraviolet radiation in the mask
aligner machinery. Where the UV radiation is able to pass through the
negative unobstructed, the photoimagable material is polymerized and
therefore will stay fixed. The areas which are protected from the UV
radiation will wash away in the subsequent developing steps. Because of
the short wavelength of the UV radiation, it is possible to obtain
conductor lines and spaces of about 0.002" each. The edges are sharp and
square, the sides are vertical and smooth. The photoimagable process is
clearly a significant improvement over the conventional prior art.
LASER CUTTING PROCESS
The second method of manufacturing this type of chip inductor uses a laser
beam, particularly an excimer laser beam that has an ultraviolet output at
248 nanometers wavelength.
This alternate method for manufacturing the high frequency chip inductor on
ceramic allows higher inductance values by making line widths and spaces
approximately 0.001". Such fine lines permit more loops and, thus, higher
inductances. This method consists of using the ultraviolet output of an
excimer laser to cut a planar spiral coil pattern in conductor material
that is in the form of a flat layer on a substrate. However, rather than
making linear cuts with a fine focused, scanned beam (as would be done in
a typical industrial metal cutting operation), a beam is projected with a
large cross section, approximately 4 cm by 3 cm, through a metal "stencil"
mask that contains the coil pattern. This transforms the beam's uniformly
intense cross section into a cross section containing the coil pattern.
The transformed beam is then optically focused onto a blank metal target
into which the coil pattern is burned or permanently transfixed by the
process of ablation. The UV excimer laser has a pulsed output so that
single or multiple pulses will be required to produce a clean and clear
coil pattern in the conductor material. The number of pulses required will
increase with the thickness of the conductor. Suitable conductor targets
on ceramic can be made by thickfilm printing, thinfilm deposition, or by
bonding metal foil to the ceramic surface.
Both the photoimagable process and the laser process yield conductor cross
sections that are more rectangular than conventional thick film screen
printing. FIG. 6A shows the typical cross section of a conductor that is
thick film screen printed. FIG. 6B shows the improved cross section of
conductors formed by laser cutting or by the photoimagable process. The
more rectangular cross section of FIG. 6B gives a desirable reduction of
the skin effect in the conductor, resulting in improved Q. The cost of the
photoimagable process is comparable economically to the conventional thick
film process, thus it is possible to meet the cost reduction goal of this
invention.
As indicated heretofore, Q is a measure of quality in inductors. It is the
ratio of inductive reactance to the sum of all resistive losses. Inductive
reactance is desirable.
Resistive losses, one of which is the skin effect, are undesirable.
As previously discussed, the skin effect is the tendency for alternating
current to flow near the surface of conductors in lieu of flowing in a
manner as to utilize the entire cross sectional area of conductors. This
phenomenon causes the resistance of the conductor to increase, thus
reducing Q. When the conductor cross section includes sharpened areas at
each side, the skin effect is increased.
The important difference between the two cross sections is that the cross
section of FIG. 6B does not include the sharpened areas at its sides which
appear in the cross section of FIG. 6A.
As indicated before, the laser system disclosed in U.S. Pat. No. 5,091,286
is of the type which can be used in this invention. Such an excimer system
is available from Lambda Physick, Inc., 289 Great Road, Acton, Mass.
01720, or the Lumonics Corp., Kanata, Ontario, Canada.
In summary, the thick film conductor material is screen printed onto a
ceramic substrate to a thickness of 15 to 25 microns and is then dried. At
this point, there are two options: to either laser cut the conductor
material in its green (or unfired) state or to sinter (fire) it first and
then laser cut the coil. It turns out that UV laser radiation can cut into
the conductor material regardless of the state it is in.
The advantage of cutting the conductor in the green state is that less
laser energy is reflected by the surface so that more energy is
incorporated into the cutting action. The net result is fewer laser pulses
would be needed to completely cut a coil. However, a disadvantage to the
green state is that the coils are very fragile and will rub off easily.
Cutting the fired state, on the other hand, where the conductor is much
more durable, will require more pulses because the conductor material is
more reflective and allows less of the incident UV energy to interact with
the metal.
The conductor material can be any of a variety of commercially available
thick film inks, but the preferred material is silver because of its high
conductivity and availability. Other conductor materials except gold or
copper will have conductivities that are too low and will generally make
chip inductors with inferior quality (Q). The addition of silver to the
conventional FODEL.RTM. process and the elimination of palladium therefrom
mentioned heretofore is a part of this invention.
After the conductive blank has been applied to the substrate, it is ready
to have the coil pattern cut in it. This is accomplished by the use of a
metal stencil mask that is positioned in the path of the beam. The mask
consists of a 2.25 inches square, 0.003-0.010 inch thick sheet of metal,
usually stainless steel or beryllium copper, that contains a cutout of the
desired coil pattern. The cutout can be made by either a laser cutting
process or by a wet etching process. The laser beam has a uniform cross
section prior to incidence on the mask, but upon emerging through the mask
it has a spiral coil cross section because of the optical shadowing by the
mask. The mask is a positive relative to the final product; that is, the
cutout in the mask is also cutout in the silver. The optical system
consists of a set of lenses and mirrors that direct and focus the laser
beam so as to image the coil pattern onto the metal target blank. Focusing
the laser beam results in an increase in the laser power density to levels
that can easily ablate the metallization of the target. Usually the
focused image is about a factor of 8.times. smaller than the mask pattern.
If the mask is thin (i.e., approximately 0.003 inches thick) and if there
are more than just a few of loops in the mask, the mask may become rather
flimsy. In this case the loops may deviate from the plane of the mask and
will most likely result in portions of the image being out of focus. The
cut coils will then tend to have less sharpness and lower line definition.
The net result will more than likely be inductors of lower Q. Loop
supports or hangers will keep the loops planar, but they will show up in
the cut coil because the UV wavelengths do an excellent job of reproducing
all the details found in the mask. One way around this potential problem
is to use a mask whose thickness is sufficient to keep the coil pattern
rigid. This can be accomplished by a 0.010 inch thick stainless steel.
Such a mask can be chemically etched.
Generally, any rare gas-halide excimer laser with emission in the
ultraviolet range is suitable for cutting the coil pattern; however, the
krypton fluoride (KrF) excimer laser, with it's output at 248 nanometers,
appears to make cuts of the best quality. The ultraviolet radiation
removes material through the process of ablation, in which thin layers
(one to several microns) of the metal thickness are removed with each
laser pulse. Multiple pulses are usually required to cut completely
through 1 or 2 mil thick metal film.
From the foregoing it is seen that a thick film low value high frequency
inductor having high quality (Q) is made possible with resistive losses
and skin effects substantially reduced by creating a rectangular cross
section in the conductor trade of the coil.
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